KO2: realization of orbital ordering in a p-orbital system

Thermally and Optically Activated Migration of Charge Carriers in Alkali Metal Sesquioxides

The alkali metal sesquioxides A 4 O 6 (A = K, Rb, Cs) are mixed-valent with respect to oxygen and display several degrees of electronic and structural freedom which give rise to diverse transport and ordering processes. We report on analyses of the respective underlying excitations by diffuse reflectance spectroscopy and thermally activated electron transport. Backed by DFT based band structure calculations we identify three possible mechanisms, inter valence charge transfer from peroxide to superoxide, excitation across the Jahn-Teller gap of tilted superoxide anions and polaron migration. The activation energies as found by the three different approaches are in a rather narrow range of 0.62 - 0.89 eV for Rb 4 O 6 and 0.49 - 0.65 eV for Cs 4 O 6 , confirming opacity in the full range of visible light. The effect of the phase transition from cubic to tetragonal as demonstrated for the caesium representative corresponds to a marginal shift to higher activation energy .

Quenching of magnetism in NaO2 due to electrostatic interaction induced partial orbital ordering

The transport properties of sodium superoxide (NaO₂) are governed by the transfer of charge between O₂^- complexes. Although it goes through a plethora of structural phase transitions, its electronic and magnetic ground state remains shrouded in mystery. In this work, we perform first-principles density functional theory (DFT) calculations to understand the relationship between electronic structure and the reason for the non-observation of an antiferromagnetic (AFM) ground state in NaO₂ vis-a-vis in KO₂. In the cubic phase, uniform < Na-O-Na bond angles result in high symmetry and hence degeneracy in the O-2p orbitals. The freely rotating O₂^- molecules result in orbital degeneracy and hence paramagnetism at room temperature. Although the degeneracy between the bonding and anti-bonding orbitals of O₂ dimers is lifted in the pyrite phase, the degeneracy between σ (σ^*) and π (π^*) states is still maintained and hence orbital degeneracy is partially lifted as the dimers are restricted to four directions now. The O-π^* states are localized in such a manner that results in a substantial magnetic moment in the π^*-orbital. The < O-Na-O bond angle (= 180°) in the c-axis facilitates a superexchange mechanism and thereby the system should be AFM in the pyrite phase. In the marcasite phase, the O-atoms are aligned parallel in alternative planes. The preservation of degeneracy among the two π^* orbitals leading to only long-range orbital ordering negates any chance of quasi-one-dimensional AFM spin chains in NaO₂. The difference in magnetic ground states of NaO₂ and KO₂ arises due to the difference in the electrostatic repulsion between electrons of Na⁺ and K⁺ ions with the O₂^- dimers.

Probing the Electronic Structure of Hybrid Perovskites in the Orientationally Disordered Cubic Phase

Hybrid organic-inorganic lead halide perovskites are projected as new generation photovoltaic and optoelectronic materials with improved efficiencies. However, their electronic structure so far remains poorly understood, particularly in the orientationally disordered cubic phase. We performed electronic structure investigations using angle-resolved photoemission spectroscopy on two prototypical samples (MAPbBr₃ and MAPbCl₃) in their cubic phase, and the results are compared with the calculations within two theoretical models where MA⁺ is orientationally (1) disordered (MA⁺ ion is replaced by spherically symmetric Cs⁺ ion) and (2) ordered (MA oriented along (100) direction) but keeping the symmetry of the unit cell cubic. Degeneracy of the valence bands and behavior of constant energy contours are consistent with model 1, which supports strongly the disordered nature of the orientation of the MA⁺ ions in the cubic phase. Band structure calculations also reveal that spin-orbit coupling induced Rashba splitting is suppressed by the orientational disorder.

Phonon-Modulated Magnetic Interactions and Spin Tomonaga-Luttinger Liquid in the p-Orbital Antiferromagnet CsO2

The magnetic response of antiferromagnetic CsO2, coming from the p-orbital S=1/2 spins of anionic O2(-) molecules, is followed by 133Cs nuclear magnetic resonance across the structural phase transition occurring at T(s1)=61  K on cooling. Above T(s1), where spins form a square magnetic lattice, we observe a huge, nonmonotonic temperature dependence of the exchange coupling originating from thermal librations of O2(-) molecules. Below T(s1), where antiferromagnetic spin chains are formed as a result of p-orbital ordering, we observe a spin Tomonaga-Luttinger-liquid behavior of spin dynamics. These two interesting phenomena, which provide rare simple manifestations of the coupling between spin, lattice, and orbital degrees of freedom, establish CsO2 as a model system for molecular solids.

DFT and two-dimensional correlation analysis for evaluating the oxygen defect mechanism of low-density 4f (or 5f) elements interacting with Ca-Mt

Existence states of f-shell electrons in Ca-Mt is calculatedviaDFT with 2D-CA techniques. Reasons of f-shell electrons influencing on electronic and optical properties are discussed. Electronic transitions are systematically analyzed.

d0 ferromagnetic interface between nonmagnetic perovskites

We use computational and experimental methods to study d(0) ferromagnetism at a charge-imbalanced interface between two perovskites. In SrTiO(3)/KTaO(3) superlattice calculations, the charge imbalance introduces holes in the SrTiO(3) layer, inducing a d(0) ferromagnetic half-metallic 2D hole gas at the interface oxygen 2p orbitals. The charge imbalance overrides doping by vacancies at realistic concentrations. Varying the constituent materials shows ferromagnetism to be a general property of hole-type d(0) perovskite interfaces. Atomically sharp epitaxial d(0) SrTiO(3)/KTaO(3), SrTiO(3)/KNbO(3), and SrTiO(3)/NaNbO(3) interfaces are found to exhibit ferromagnetic hysteresis at room temperature. We suggest that the behavior is due to the high density of states and exchange coupling at the oxygen t(1g) band in comparison with the more studied d band t(2g) symmetry electron gas.

Antiferromagnetic S=1/2 spin chain driven by p-orbital ordering in CsO2

We demonstrate, using a combination of experiment and density functional theory, that orbital ordering drives the formation of a one-dimensional (1D) S=1/2 antiferromagnetic spin chain in the 3D rocksalt structure of cesium superoxide (CsO2). The magnetic superoxide anion (O2(-)) exhibits degeneracy of its 2p-derived molecular orbitals, which is lifted by a structural distortion on cooling. A spin chain is then formed by zigzag ordering of the half-filled superoxide orbitals, promoting a superexchange pathway mediated by the p(z) orbitals of Cs(+) along only one crystal direction. This scenario is analogous to the 3d-orbital-driven spin chain found in the perovskite KCuF3 and is the first example of an inorganic quantum spin system with unpaired p electrons.

Ferromagnetic spin coupling of 2p impurities in band insulators stabilized by an intersite Coulomb interaction: nitrogen-doped MgO

For a nitrogen dimer in insulating MgO, a ferromagnetic coupling between spin-polarized 2p holes is revealed by calculations based on the density functional theory amended by an on-site Coulomb interaction and corroborated by the Hubbard model. It is shown that the ferromagnetic coupling is facilitated by a T-shaped orbital arrangement of the 2p holes, which is in its turn controlled by an intersite Coulomb interaction due to the directionality of the p orbitals. We thus conjecture that this interaction is an important ingredient of ferromagnetism in band insulators with 2p dopants.